1. Field of the Invention
The present invention relates to a backlight unit for a flat panel display and a flat panel display apparatus having the same and, more particularly, to a backlight unit for a flat panel display designed to alleviate or remove color separation by a diffraction grating and a flat panel display apparatus having the same.
2. Description of the Related Art
Unlike self-emissive flat panel displays, non-emissive flat panel displays such as liquid crystal display (LCD) panels need external light to produce an image. Thus, a backlight unit is located behind a non-emissive flat panel display and illuminates light on the flat panel display such as an LCD panel in order to produce an image. The backlight unit for the flat panel display is used as a backlight unit for an LCD device or a surface light source system such as an illuminating sign.
Backlight units are classified into direct light type backlight units and edge light type backlight units according to the position of a light source. A point light source having an approximately point-shaped light-emitting portion or a linear light source having a linear light-emitting portion disposed along one direction may be used as a light source for an edge light type backlight unit. Representative examples of the linear light source and point light source are a cold cathode fluorescent lamp (CCFL) having two electrodes at opposite ends within a tube and a light emitting diode (LED) (or laser diode), respectively.
Korean Laid-open Patent Publication No. 2003-4021 discloses a backlight unit for a flat panel display using a planar hologram filed by an applicant of the present invention. FIG. 1 is a schematic cross-sectional view of the disclosed backlight unit. Referring to FIG. 1, the backlight unit includes a light source 51 disposed within a housing 55, a light guide panel (LGP) 20 guiding light emitted by the light source 51 by the use of total reflection, a reflective member 31 that is disposed below the LGP 20 and reflects upward light escaping from the LGP 20, and a transmissive diffusion sheet 11 that is disposed above the LGP 20 and widely diffuses light escaping upward from the LGP 20. The LGP 20 has a holographic pattern 21 repeated continuously with a grating period P at the bottom thereof. The light incident on the LGP 20 is totally internally reflected into the LGP 20 by top and bottom surfaces thereof and propagates along the LGP 20. Some of light injected onto the holographic pattern 21 is diffracted downward onto the reflective member 31 by the holographic pattern 21 and reflected back into the LGP 20. When a white light source is used, white light having multiple wavelengths is separated into single-color light beams having different exit angles θt according to their different wavelengths, e.g., red (R) green (G), and blue (B) light beams as it passes through the holographic pattern 21.
The color separation occurs due to the characteristics of the holographic pattern 21. That is, because the light incident on the holographic pattern 21 is diffracted at different angles depending on the wavelength of the incident light, the white light of mixed wavelengths is separated into its component colors at different exit angles θt according to wavelength.Θt=sin−1[mλ/p+nθi]  (1)where m is a diffraction order, λ is the wavelength of incident light, P is a grating period of a holographic pattern, θt and θi are respectively exit angle and incident angle of light with respect to the holographic pattern, and n is a refractive index of an LGP as medium characteristics of an LGP having the holographic pattern. As evident from Equation (1), since the angle of light exiting the holographic pattern varies with the wavelength of incident light, white light having incident angle θi is separated into component colors according to wavelength as it passes through the holographic pattern.
FIG. 2 shows profiles of distribution of exit angle θt with respect to incident angle θi for individual blue (b), green (g), and red (r) color light. Profiles b, g, and r show changes in exit angles θt of blue, green, and red light. As evident from FIG. 2, the exit angle θt increases proportionally to incident angle θi of individual RGB color light. For example, when white light enters the LGP 20 at about 60° as shown in FIG. 1, green light may exit the LGP 20 at angle of about 0° while blue and red light exit the LGP 20 obliquely at angles of −7° and +7° with respect to vertically exiting green light, respectively.
Different colors are sensed by human eyes according to the direction in which an image is observed due to color separation caused by single color light exiting at different angles. For example, when a display plane is observed from the front, green may be more strongly sensed than other colors. When the display plane is observed obliquely away from a vertical axis, red and blue are more strongly sensed. Thus, the color separation leads to degradation in image quality.